In internal combustion engines, the exhaust gas turbo-charger is used to increase the power. in many applications with variable engine speed (for example ship and vehicle propulsion), it is desirable to have a high boost pressure at part speed. Various arrangements are known for relatively increasing the boost pressure at partial-load compared with boost at the full load operating point, such as the use of a turbine with especially narrow flow cross-sections such an arrangement makes it possible to increase the boost pressure over the whole operating range. In order to ensure that the boost pressure at full load does not exceed a permissible value, either a part of the boost air is blown off to atmosphere downstream of the compressor or part of the exhaust gases is blown off to atmosphere upstream of the turbine at high power levels. The disadvantage of this solution is that blowing off high pressure air or gas in effect lowers turbo-charger efficiency and consequently increases fuel consumption.
Another possibility for achieving a relative increase in the boost pressure at partial load is provided by the use of adjustable turbine guide vanes. Adjustable guide vanes make it possible to have a large turbine cross-section at full load and a smaller turbine cross-section at partial load. The disadvantages of this solution is that the adjustment of a large number of guide vanes (18 to 36 vanes, for example) is mechanically very complicated and has poor operational reliability, particularly when the turbo-charger turbine is driven by the exhaust gases of a diesel engine operating on heavy oil. The danger exists of the movable guide vanes being eroded by hard soot particles or the movement of the guide vanes being prevented by deposits of incompletely burned fuel particles.
Exhaust gas turbines are known from DE No. 2,934,041 C2 and DE No. 3,346,472 A1 in which various ducts undertake the flow of the rotor blades as a function of load. These turbines, however, all have vaneless guide arrangements.
A turbine known from EP No. 0,093,462 A1 is equipped with an axially adjustable annular slide for changing the turbine cross-section, the annular slide being located between the turbine distributor and the turbine guide vanes. The result, however, is that the exhaust gas flow is led to the rotor blade tips at part load, i.e. to the point where the centrifugal effect of the exhaust gas flow is at a maximum. Consequently there is, the imminent danger that the small flow cross-section will be plastered up by soot particles.
This design also involves the end surface of the annular slide being designed so as to be free from separation which, when the design is based on a variable exhaust gas flow, can lead to non-optimum design compromises.